I. Field of the Invention
This invention relates generally to a device and method for pacing a patient's heart and more particularly to a device and method for improving the hemodynamic performance of patients suffering from heart failure through dual chamber pacing. The patient in need of improving hemodynamic performance may suffer from, for example, congestive heart failure (CHF), or other heart failure requiring pacing even though intrinsic PR conduction is present. The device includes a means for tracking an R-wave associated with intrinsic conduction in a ventricle of the patient's heart. The method includes tracking a sensed R-wave for a predetermined RV delay interval and then stimulating the ventricle if a P-wave is sensed during the preset simultaneous Post Ventricular Atrial Refractory Period (PVARP) interval.
II. Discussion of the Related Art
Typically, a patient suffering from a higher degree of AV-block or an AV conduction disorder is implanted with a conventional atrial tracking (DDD or VDD) pacemaker suited for pacing the ventricle. Such a pacemaker is designed to pace the ventricle after a pre-set AV delay, synchronous with the intrinsic atrial rate. The purpose of these pacemakers is to make sure that heart beats are properly timed and not omitted. Intrinsic rhythm is favorable over paced rhythm for both hemodynamic and economic (battery conservation) reasons. It is also important and necessary to prevent pacing the ventricle during the vulnerable period for inducing arrhythmia immediately following an intrinsic ventricular beat. Hence, ventricular pacing is inhibited when intrinsic conduction from the SA node causes an R-wave to be sensed prior to the scheduled ventricular pace.
Patients suffering from congestive heart failure (CHF), for example, either do not exhibit heart block at all or often only suffer from a first degree AV-block or a slightly prolonged delay interval between the depolarization of the atrium and the depolarization of the ventricle. Recent research has shown that acute hemodynamic performance, exercise tolerance and quality of life of CHF patients can be improved by a pre-excitation of the ventricles with pacing following normal sinus beats. Patients benefiting from pre-excitation of the ventricles experience a return of heart failure symptoms immediately upon omission of pre-excitation pacing. Thus, for these patients, it is important that the paced pre-excitation of the ventricle be performed continuously in order to improve the contraction pattern, even though intrinsic beats would occur slightly later if there were no pacing. When pacing a patient suffering from CHF, it is highly undesirable to omit pacing when it is supposed to occur.
When pacing the heart of a CHF patient having normal intrinsic (PR) conduction, although continuous pacing is desirable, use of a conventional atrial tracking dual chamber pacemaker completely inhibits ventricular pacing when the intrinsic atrial rate (hereinafter the sinus rate) rises above a preprogrammed atrial maximum tracking rate (AMTR). These pacemakers also exhibit an undesirable pronounced pacing hysteresis, wherein ventricular pacing is not resumed until the sinus rate falls below a ventricular inhibition threshold rate (VIR). Also, in these pacemakers, a premature ventricular contraction (PVC) may inhibit ventricular pacing when the sinus rate exceeds the VIR.
It would be advantageous to eliminate the pacing hysteresis, while extending the limit or MTR for pacing of the ventricle. However, this is not possible with the conventional atrial tracking multi-chamber pacemaker. A CHF patient, for example, has an elevated sinus rate in order to maintain normal cardiac output despite reduced cardiac pumping efficiency and, therefore, requires a high atrial MTR. Together with an elevated sinus rate, many such patients also have prolonged PR intervals and correspondingly prolonged retrograde conduction intervals requiring long PVARP intervals. The required long PVARP intervals prevent tracking of retrograde P-waves, thereby reducing pacemaker mediated tachycardia (PMT). In conventional atrial tracking pacemakers, the highest allowable atrial MTR is determined in part by the length of the PVARP interval, which may limit the atrial MTR to a rate that is below the normal range of sinus rates in the CHF patient. Hence, use of a conventional atrial tracking dual chamber pacemaker would not allow continuous ventricular pacing above the atrial MTR. Therefore, there is a need for a dual chamber pacemaker and a method of operating the same that may be used to pace the failing heart of a patient having intrinsic conduction, wherein the pacemaker provides for continuous pacing of the ventricle at a sinus rate that exceeds an atrial maximum tracking rate and does not exhibit pacing hysteresis below the MTR. The present invention addresses this need.